Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. For the formation of positive silver images, a latent image contained in an exposed photosensitive silver halide emulsion is developed and almost concurrently therewith a soluble silver complex is obtained by reaction of a silver halide solvent with unexposed and undeveloped silver halide of said emulsion. The photosensitive silver halide emulsion is developed with a processing composition which may be spread between the photosensitive element comprising the silver halide emulsion and a second element which may comprise suitable silver precipitating layer. The processing composition effects development of the latent image in the emulsion, and, substantially contemporaneous therewith, forms a soluble silver complex, for example, a thiosulfate or thiocyanate, with undeveloped silver halide. This soluble silver complex is at least in part transported in the direction of the print receiving layer and the silver thereof is precipitated in the silver precipitating element to form a positive image. Procedures of this desription are disclosed, for example, in U.S. Pat. No. 2,543,181 issued to Edwin H. Land. See also Edwin H. Land, One Step Photography, Photographic Journal, Section A, pgs. 7-15, January 1950.
U.S. Pat. No. 3,674,482 issued July 4, 1972, is directed to a silver diffusion transfer film unit which comprises a support carrying on one surface, in order, a layer containing silver precipitating nuclei, an inert non-nuclei-containing protective layer and a layer containing a photosensitive silver halide emulsion. The purpose of the non-nuclei-containing protective layer is to provide a layer over the transferred silver image after the emulsion layer has been removed subsequent to processing which protective layer will then be the outermost layer. The material for the protective layer is one which is readily permeable to the processing composition and which will not provide sites for the nucleation of the silver forming the transferred image. A particularly preferred material employed as a protective layer comprises chitosan (deacetylated chitin).
U.S. Pat. No. 4,056,392 issued Nov. 1, 1977 is directed to a diffusion transfer film unit which comprises, in order, an additive color screen, a layer comprising silver precipitating nuclei, a layer comprising a water-soluble cupric salt and a compound selected from the group consisting of chitosan and 2-amino-2-deoxyglucose, and a photosensitive silver halide emulsion layer. By employing a water-soluble copper salt in the chitosan protective layer an increase in D.sub.max is achieved with substantially no adverse effect on D.sub.min compared to a protective layer composed of chitosan alone.
U.S. Pat. No. 3,677,753, issued July 18, 1972, is directed to a photographic process wherein an exposed and processed film unit of the type decribed in U.S. Pat. No. 3,674,482 is contacted with a revolving roller having a coating thereon to which the photosensitive layer is more adherent than the adjacent layer of the film unit thereby detaching the photosensitive layer from the film unit.
U.S. Pat. No. 4,359,518 issued Nov. 16, 1982 is directed to a method which comprises exposing a diffusion transfer film unit which includes an image-receiving layer and a photosensitive silver halide emulsion layer; disposing a liquid processing composition intermediate a stripping sheet and the outermost layer of the film unit wherein the stripping sheet comprises a support carrying, in order, a water-absorbing layer and a timing layer capable of conversion from substantial liquid processing composition impermeability to a condition of substantial liquid processing composition permeability, whereby said timing layer forms a bond between the outermost layer of the film unit and the stripping sheet, and detaching the stripping sheet and the photosensitive layer of the film unit which has adherred to the stripping sheet from the rest of the film unit.
U.S. Pat. No. 3,930,864 issued Jan. 6, 1976 is directed to a photographic assemblage which employs a scavenger layer capable of immobilizing byproducts of processing reactions which may affect image quality in the image-receiving layer.
U.S. Pat. No. 4,370,045 issued Jan. 25, 1983 is directed to an apparatus for processing an exposed roll of self-developing or instant-type transparency film. One aspect of the invention employs a container of processing liquid and a processing liquid dispenser. In operation, the processing composition flows from the container into the dispenser. Sheet material passes beneath the dispenser acquiring a coating of the processing liquid on the surface thereof. That coated surface is then moved into engagement with the emulsion side of the exposed film and directed between a pair of rollers to form a laminate. After the desired period of processing, the processed film and the sheet are separated with the processed silver halide emulsion layer remaining with the sheet and the film unit now consisting essentially of the positive transparency image.
The above-mentioned patents which employ a stripping step are involved with the removal of the exposed and processed silver halide emulsion layer. The desired product, therefore, consists of a positive silver image. By means of the present invention, a negative image of enhanced quality is obtained.